Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for several decades. To achieve increased extended drain intervals, resistance to thermal and oxidative degradation, lower oil consumption, low volatility, and greater total base number (TBN) retention, engine lubricant formulations have evolved from mineral based oils (API Group I) to synthetic base oils, such as high viscosity polyalphaolefins (PAO) (API Group IV). PAOs have several advantages over mineral based oils, such as narrow boiling ranges, low pour points, and high viscosity indexes for all grades having a kinematic viscosity at 100° C. of 4 cSt (mm2/s) or greater. Formulated PAO base stocks, sometimes combined with additives, offer excellent performance over a wide range of lubricating properties. The advantages of PAO base stocks have lead to an increased use of PAO in several different applications as synthetic lubricants for greases, automotive lubricants, marine lubricants, aerospace lubricants, automotive oils, transmission oils, industrial gear oils, circulating oils, turbine oils, extreme pressure gear oils, refrigeration compressor lubricants, air compressor oils, and hydraulic fluids. The amount of PAO produced to meet the demand for these applications continues to increase.
Catalytic oligomerization of alpha-olefin monomer (α-monomer) is a known technique for manufacturing PAO base stocks useful as lubricants. PAOs are formed from α-monomer or a mixture of α-monomers containing from 2 to 20 carbon atoms. Typically a catalyst system, such as a Ziegler-Natta catalyst of the triethylaluminum/titanium tetrachloride type, alkyl-aluminum halide-alkoxide-zirconium halide type or a boron trifluoride-base Friedel-Craft, is used to polymerize the α-monomer.
U.S. Publication No. 2007/0225534 discloses a method of making a PAO using tetradecene and particularly mixtures comprising 1-hexene, 1-decene, 1-dodecene, and 1-tetradecene, characterized by a low viscosity, kinematic viscosity at 100° C. of from about 4 to about 12 cSt, and excellent cold temperature properties, using a promoter system comprising an alcohol and an ester. This reaction may be carried out continuously in one continuously-stirred tank reactor or in a series of at least two continuously-stirred tank reactors.
U.S. Pat. No. 4,469,910 discloses a process for oligomerizing alpha-olefin monomers comprising contacting in a reaction zone under reaction conditions: (a) alpha-olefin monomers having at least three carbon atoms; (b) an aluminum compound having the formula R3Al, wherein R is an hydrocarbyl group and (c) a cocatalyst which is hydrocarbyl halide having at least one halogen group reactive with said aluminum compound, elemental bromine or elemental iodine.
A significant problem in the manufacture of synthetic lubricants is the difficulty in producing lubricants of a preferred high viscosity range in good yield without excessive catalyst deactivation. It is difficult to directly produce higher viscosity range lubricants without incurring lower yields due to the production of non-lubricant range materials. Thus, methods to control molecular weight of lubricants in the oligomerization step are sought after in the art to overcome the problems in the manufacture of, particularly, higher viscosity lubricants. Continued improvement in the production of PAO is needed with greater efficiencies and lower costs.